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Event Shapes in Soft-Collinear Effective Theory

Final Report Summary - ESSCET (Event Shapes in Soft-Collinear Effective Theory)

Description of project objectives

• Theoretical description of event-shape distributions at leptonic colliders including higher-order perturbative resummation and power corrections.
• Determination of the strong coupling from fits of the theoretical distribution to experimental data.
• Determination of leading power correction on the tail of the distribution.
• Determination of the non-perturbative shape function.
• Study of heavy quark mass effects in event-shape distributions.
• Application of the methods listed above to hadronic colliders.

Description of the work performed since the beginning of the project

a) Published work

• We have developed the most sophisticated description of event-shape distributions, including higher-order resummation and matrix elements, plus power corrections, as well as other smaller corrections such as heavy quark mass effects, QED effects and the axial-singlet anomaly.

• Our determination of the strong coupling (in collaboration with researchers at MPI, MIT and the University of Arizona) is one of the most precise in the world, competing directly with Lattice QCD results. Our best-fit value is considerably lower than the world average, but however compatible with independent determinations such as Deep-Inelastic-Scattering analyses or other event-shape studies. We have confirmed the agreement between the determinations from Thrust distributions and the Thrust first moment, and our preliminary results seem to indicate nice agreement with the C-parameter distribution.

• Our work on universality and power corrections (in collaboration with senior researchers at MIT) has demonstrated that the naïve OPE used in many analyses is only valid at tree level, since the leading power correction has non-trivial running and matching. This work has also identified relevant classes of measurements with common power corrections.

• On the other hand, with the scientist in charge, we have shown from first principles that the most general structure of oriented event-shape distributions (that is, when measuring the angle between the Thrust axis and the leptonic beam and the event shape simultaneously) is very simple, consisting in two structures. We have also shown that only one of the two structures is afflicted by large Sudakov logs, that can be resummed in SCET to all orders. Finally we have computed the oriented distribution to second order in perturbation theory.

b) Work in progress and final expected results

• We are improving the theoretical description of the HJM distribution in a number of ways. Firstly, we have created a code that is capable of performing resummation at N3LL without any approximation and secondly we have incorporated hadron mass effects.

• We expect to achieve soon compatible determinations of the strong coupling and the leading power correction from Thrust, C-parameter (this project involves a student at MIT) and HJM (involving a senior researcher at Harvard) distributions, as well as from moments of those distributions.

• We are intensely working on including hadron mass effects in the fits for the strong coupling, a level of sophistication never achieved so far. It is particularly challenging to combine together hadron mass effects and the Rgap formalist. At the moment we have understood how to coherently intertwine these two concepts, have built the appropriate computer code and are right now analyzing the data with it. This last level of refinement will be used in Thrust, C-parameter and HJM. Results are expected in the next months (except HJM which might take longer).

• We will have very soon the computations necessary to treat heavy quark effects in a number of different cases. For instance, we already have almost full results for Thrust, HJM and C-parameter primary and secondary effects (these projects involve PhD and master students as well as postdocs and senior researchers at the University of Vienna), and are working on primary mass effects on oriented distributions (this involves the scientist in charge). All these computations will be incorporated to our computer code. Results will be produced in the next two years.

• We are working very hard in the determination of the full shape function for the Thrust distribution. In this project we have involved an undergraduate student at MIT. Results are expected along this year.

• We have started a project to determine the two-loop C-parameter soft function, involving a postdoc at IFIC (CSIC), and expect to have results during 2014.

• Finally, we expect to have a computer code capable of describing distributions of jet vetoes for vector-boson production at the LHC. This work involves a reasearcher at DESY Hamburg.

• Other future (long term) plans include: Generalization of N-jettiness to be less sensitive to the underlying event; Study of “schemes” on heavy quark event-shape distributions; Study of hadron mass effects at the light of AdS coordinates; Determination of the strong coupling using event-shape DIS data.


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